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Pegmatites are magmatic bodies consisting of centimeter to meter sized crystals and can sometimes be a source for critical economic resources such as lithium (Li). Determining the chemistry and kinetics involved in pegmatite formation may be important for understanding the element enrichment process. Here, we analyzed the Li isotope compositions of quartz crystals from the Stewart pegmatite in southern California, USA. We find large Li isotopic fractionations: >40‰ between different crystals and >20‰ from core to rim in a single crystal. Two mechanisms for these extreme fractionations were considered: rapid crystal growth rate and Rayleigh fractionation. We find that although rapid crystal growth rate (1–10 m/day) can explain elemental variations, rapid growth alone is unable to explain the most extreme isotopic fractionations. Rayleigh fractionation can account for the largest isotopic fractionations if 96–99.9% of the Li is removed from the system through crystallization of lepidolite and spodumene, but alone cannot explain the observed elemental variability in the quartz. We thus suggest that both processes operated. Trace element enrichments may be more sensitive to growth rate while Li isotope ratios may be more sensitive to changes in the isotopic composition of the pegmatitic fluids during crystallization of the quartz. [ABSTRACT FROM AUTHOR]

[Display omitted] • Banded veins in low-sulfidation epithermal deposits consist of quartz that formed through recrystallization of opal-A G. • Incipient recrystallization resulted in the formation of lepispheres consisting of opal-CT. • Continued maturation formed mosaic quartz, flamboyant quartz, and large prismatic quartz crystals with zones of feathery appearance. • Where recrystallization proceeded to completeness, textures could be misinterpreted to indicate that quartz growth occurred in open space. • Recognition of recrystallization textures has implications for paragenetic investigations and the design of fluid inclusion studies. High-grade ores in low-sulfidation epithermal deposits commonly consist of banded veins containing quartz as the most abundant gangue mineral. Previous studies suggested that at least some of the quartz has formed as a product of recrystallization from a noncrystalline silica precursor. Detailed petrographic studies confirm that high-grade veins from the <2.2 Ma McLaughlin deposit in California were originally entirely composed of opal-A G. The noncrystalline silica is isotropic in crossed-polarized light and consists of compacted microspheres that are up to several micrometers in size. In many bands of the high-grade veins, the thermodynamically unstable opal-A G has matured to quartz. The recrystallization of the noncrystalline silica resulted in the development of quartz and ore textures that mask the original conditions of vein formation. Incipient recrystallization of the opal-A G caused the formation of lepispheres consisting of opal-CT as well as the development of concentrically banded silica spheres. Continued maturation led to the growth of elongated quartz crystals or complexly shaped quartz aggregates in the cores of the concentrically banded silica spheres. Amalgamation of quartz crystals caused the development of mosaic quartz or flamboyant quartz. Ripening produced large prismatic quartz crystals, which are characterized by zones of feathery appearance. Fluid inclusions within the quartz formed through recrystallization are typically highly irregular in shape and show inconsistent liquid-to-vapor volumetric proportions, but assemblages with consistent ratios are also present. Bands rich in fluid inclusions occurring along former recrystallization fronts in prismatic quartz crystals resemble growth zones in zonal quartz. Recrystallization of the silica matrix resulted in grain coarsening of the ore minerals and encapsulation of ore minerals by quartz. In areas where recrystallization of the noncrystalline silica proceeded to completeness, the quartz textures could be easily misinterpreted to indicate that quartz growth occurred in open space with the ore minerals infilling vug spaces. Correct recognition of recrystallization textures has significant implications for paragenetic investigations on vein material from epithermal deposits as well as the design of fluid inclusion studies. [ABSTRACT FROM AUTHOR]

Kinetic calcium isotope effects can be used as growth-rate proxies for volcanic and subvolcanic minerals. Here, we analyze Ca isotopic compositions in experimental and natural samples and confirm that large kinetic effects (>2?) can occur during magmatic plagioclase crystallization. Experiments confirm theoretical predictions that disequilibrium isotope effects depend mainly on the rates for crystal growth relative to liquid phase Ca diffusivity (R/D). Plagioclase phenocrysts from the 1915 Mount Lassen rhyodacite eruption, the ~650-y-old Deadman Creek Dome eruption, and several mafic subvolcanic orbicules and plagioclase comb layers from Northern California have disequilibrium Ca isotopic compositions that suggest rapid crystal growth rates (>1 cm/y to 15 cm/y). The Ca isotope results, combined with complementary crystal-size distribution analyses, suggest that magmatic rejuvenation (and eruption) events, as reflected in crystal growth times, can be as short as ~10-3 y. Although mafic systems are predicted to have shorter magmatic rejuvenation periods, we find similarly short timescales in both mafic and silicic systems. These results are consistent with a growing body of evidence suggesting that dominantly crystalline volcanic magma reservoirs can be rapidly reactivated by the injection of fresh magma prior to eruption. By focusing on a commonmineral such as plagioclase, this approach can be applied across all major magmatic compositions, suggesting that Ca isotopes can be used as a tool for investigating the dynamics and timing of volcanic eruptions. [ABSTRACT FROM AUTHOR]

The Papoose Flat pluton in the White-Inyo Range, California, is one of the best examples of forcefully emplaced plutons within an arc crust, having internal fabrics and a contact aureole that deformed in a ductile manner. A 2-D numerical model for the thermo-rheological evolution of the pluton-wall rock system is proposed. We explore how the frequency of magma input, from instantaneous, episodic to continuous, affects magma chamber crystallinity and rheology of both the pluton and its contact aureole. We model pluton growth in the depth range of 10-13 km, which is at the brittle-ductile transition of the crust, and in the 7-4 km depth range, where the host rocks are initially brittle. For incremental growth (episodic and continuous), the pluton begins as a sill. With subsequent injections to the bottom, the pluton grows into a laccolith. Results of mid-crustal models show that the ductile region above the Papoose Flat pluton is related to thermal weakening. The ductile region during incremental growth is 100-150 m thick, matching the observed thickness. It is ten times thinner than in the instantaneous growth model. In episodic and continuous models, the upper part of the pluton is fully or quasi-crystalline throughout growth. During continuous growth, it is likely to remain ductile with potential for the development of solid-state fabrics. During episodic growth, strain rates within the pluton during each injection may become sufficiently high to cause embrittlement of magma. In no case a ductile aureole develops above the pluton at the upper-crustal level, but may develop below the pluton, which serves as thermal insulator. Thus, the pluton's floor may sag. During incremental growth, most of the pluton is below the solidus and brittle. The results suggest that large volcanic eruptions are unlikely to occur by slow pressurization of magma chambers; instead they require rapid injections of large melt volumes. [ABSTRACT FROM AUTHOR]

Dolomite [Ca,Mg(CO3)2] precipitation from supersaturated ionic solutions at Earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process. Nevertheless, recent dolomite is observed to form in hypersaline and alkaline environments. Such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures. The mechanism of microbially mediated dolomite precipitation is, however, poorly understood and it remains unclear what role microbial mediation plays in natural environments. In the study presented here, simple geochemical methods were used to assess the limitations and controls of dolomite formation in Deep Springs Lake, a highly alkaline playa lake in eastern California showing ongoing dolomite authigenesis. The sediments of Deep Springs Lake consist of unlithified, clay-fraction dolomite ooze. Based on δ18O equilibria and textural observations, dolomite precipitates from oxygenated and agitated surface brine. The Na-SO4-dominated brine contains up to 500 m m dissolved inorganic carbon whereas Mg2+ and Ca2+ concentrations are ca 1 and 0·3 m m, respectively. Precipitation in the subsurface probably is not significant because of the lack of Ca2+ (below 0·01 m m). Under such highly alkaline conditions, the effect of microbial metabolism on supersaturation by pH and alkalinity increase is negligible. A putative microbial effect could, however, support dolomite nucleation or support crystal growth by overcoming a kinetic barrier. An essential limitation on crystal growth rates imposed by the low Ca2+ and Mg2+ concentrations could favour the thermodynamically more stable carbonate phase (which is dolomite) to precipitate. This mode of unlithified dolomite ooze formation showing δ13C values near to equilibrium with atmospheric CO2 ( ca 3‰) contrasts the formation of isotopically light (organically derived), hard-lithified dolomite layers in the subsurface of some less alkaline environments. Inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in Deep Springs Lake may shed light on conditions that favoured extensive dolomite formation in alkaline Precambrian oceans, as opposed to modern oceans where dolomites only form diagenetically in organic C-rich sediments. [ABSTRACT FROM AUTHOR]

Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together. Pegmatite crystals are thought to grow rapidly, yet their growth rates and conditions are not well constrained. Here, the authors find that the trace element distributions of pegmatitic quartz crystals indicate rapid growth in highly dynamic environments, suggesting that large meter-scale crystals can be formed within days. [ABSTRACT FROM AUTHOR]

Discusses the development of a method to control the growth of crystals by graduate students of University of California in Berkeley. Use of two surface surfactants with similar non-polar pairs; Mechanics of making cadmium-selenium teardrops; Rods; Four-armed tetrapods.